Communication pubs.acs.org/cm
Reaction of Methane with Bulk Intermetallics Containing Iron Clusters Yields Carbon Nanotubes Patricia C. Tucker, Adrian Lita, and Susan E. Latturner* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *
KEYWORDS: carbon nanotubes, intermetallics, iron clusters, disproportionation
C
and Si atoms. Ce21Fe8Si7C12 contains Fe4 tetrahedra capped on each side by carbon atoms; these Fe4C6 units are surrounded by a Ce/Si network. Ce33Fe14B25C34 can be viewed as a bodycentered cubic array of Fe14B clusters capped with borocarbide chains and surrounded by Ce3+ cations. La6Fe10Al3Si has 2D slabs of linked Fe icosahedra separated by La-rich layers. The ability of these phases to convert methane to CNT appears to be directly linked to the dimensionality and extent of Fe−Fe bonding of the iron building block. Methane was reacted with the intermetallic phases depicted in Figure 1 in a CVD process. The syntheses of Ce33Fe14B25C34, La6Fe10Al3Si, and Y5Mg5Fe4Al12Si6 were carried out as described previously;11,13,14 crystal growth of Ce21Fe8Si7C12 from Ce/Co flux followed the method reported for its lanthanum analog La21Fe8Sn7C12 (see the Supporting Information).12 Chemical vapor deposition experiments were carried out using a tube furnace and a 2.5 cm diameter quartz tube. Source gases were Airgas 100% methane (UN1971) and Airgas UHP nitrogen (UN1066). The intermetallic phases are slightly air-sensitive, so care was taken to avoid exposure to trace oxygen or water, particularly at high temperatures. Large (1−2 mm diameter) crystal samples of the four compounds were placed into individual alumina boats positioned in the center of the quartz tube. UHP nitrogen was directed through the tube as the temperature was ramped from room temperature to 690 °C in 12 °C/min. Once the process temperature was reached, methane gas was directed over the intermetallic crystals at a flow rate of 1 mL/min, for up to 120 min. After the set reaction time, nitrogen gas was reintroduced and the furnace cooled to room temperature. Numerous trials were carried out to investigate the effects of reaction time and temperature on the growth of CNT. The resulting growth on the surface of the catalyst crystals was imaged with SEM, TEM, Raman spectroscopy, and XPS (details in the Supporting Information). Ce33Fe14B25C34 appears to be the most reactive phase studied in this work. The SEM and TEM images obtained for growth on this phase at a reaction temperature of 690 °C are shown in Figure 2 (reactions at lower temperature of 590 °C did not produce any product). For short reaction times at 690 °C (25 min), there is no clear evidence of CNT formation. The darker
arbon nanotubes (CNTs) are of great interest for a wide variety of applications because of their high strength and one-dimensional electronic properties.1,2 A common synthesis method for CNTs is chemical vapor deposition growth.3−5 In this process, a gaseous carbon feedstock (such as methane, ethylene, or acetylene) is directed over a bed of catalyst at elevated temperature. The mechanism of conversion of hydrocarbon to carbon nanotubes is unclear, although the procedure must involve cleaving of C−H bonds to form hydrogen and carbon, dissolution of carbon into the catalyst particles, and crystallization of carbon from the supersaturated carbide solid solution as nanotubes. Many different metals have been explored as catalysts, but the majority of work in this field has focused on iron, cobalt, or nickel nanoparticles.6 These were originally thought to be the only catalysts capable of yielding single walled carbon nanotubes (SWCNT), although recent work using Au and Pd nanoparticles has proven otherwise.7,8 Binary alloys such as LaNi5 and Fe/Zr alloys have also been explored, although the studies concluded that most of the catalytic activity occurred at islands of Ni or Fe, respectively.9,10 We have found that complex iron-containing intermetallics in bulk crystalline form can also react with methane to form CNT. Pha ses such as Y 5 Mg 5 F e 4 Al 1 2 Si 6 , Ce 2 1 Fe 8 Si 7 C 1 2 , Ce33Fe14B25C34, and La6Fe10Al3Si can be grown from metal fluxes as large crystals.11−14 The structures of these compounds contain iron building blocks of varying sizes, as shown in Figure 1. Y5Mg5Fe4Al12Si6 does not feature any Fe−Fe bonding; instead, the iron atoms in the structure are surrounded by Al
Received: February 2, 2013 Revised: April 13, 2013
Figure 1. Iron-containing intermetallics explored in reactions with methane. Iron structural units shown in red. © XXXX American Chemical Society
A
dx.doi.org/10.1021/cm400422c | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Communication
Figure 3. Raman spectra of CNT growth on Ce33Fe14B25C34 after varying amounts of reaction time with methane at 690 °C, and associated TEM images.
possibly MWCNTs. After 45 min, SWCNT growth becomes apparent in both the TEM images and Raman spectra. Observation of characteristic radial breathing modes in the 150−350 cm−1 range and lack of extraneous peaks in the XPS spectrum also support formation of SWCNT (see the Supporting Information, Figures S2 and S3).15−17 Elemental analysis using SEM-EDS on Ce33Fe14B25C34 crystals after reaction with methane indicated that the Ce/Fe ratio on the surface had increased from the 7:3 ratio expected from the stoichiometry (and observed for crystals before the CVD process). The crystals were also analyzed by X-ray diffraction. The majority of the samples had become amorphous. Larger crystals still displayed weak diffraction peaks which indicated conversion to a new phase. The peaks could be fit to a hexagonal primitive unit cell with a = 5.45 Å and c = 16.86 Å. This cell matches CeNi3 and Lu5Ni19B6 structure types and could possibly indicate formation of ironcontaining analogs such as CeFe3 or Ce5Fe19B6.18 Neither of these phases have been reported previously; CeFe3 is not known, but Ce2Mn3Ni3 exists with the CeNi3 structure, indicating that a metastable (or doped or hydrided) CeFe3 may also exhibit this structure type.19 These data indicate that the reaction of methane with Ce33Fe14B25C34 to form CNTs is not a catalytic process, but instead results in the degradation of the structure. Degradation to a more catalytically active species has been observed in studies of micrometer-sized NdNi5 as a CNT growth catalyst; interaction with methane was observed to cause nanodusting of the intermetallic and conversion to more catalytically active nanosized particles.20 Degradation of Ce33Fe14B25C34 may induce agglomeration of the Fe14 clusters near the surface regions of the crystal into iron nanoparticles which catalyze the formation of CNT via a tip growth mechanism; this would remove iron from the crystal surface and thus increase the Ce/ Fe ratio. The hydrogen byproduct of the reaction with methane will be absorbed by the Ce33Fe14B25C34 bulk, inducing
Figure 2. SEM (left column) and TEM (right column) images of CNT growth on the surface of Ce33Fe14B25C34 crystals after reaction with methane at 690 °C. (a) 25, (b) 30, (c) 45, and (d) 60 min of reaction time.
regions on the SEM image suggest preliminary graphitic growth on the surface of the crystal (supported by Raman data, vide infra). After 30 and 45 min, CNT growth is evident coating the sides of the crystal. After 1 h of reaction with methane, the TEM image clearly shows long SWCNT growing from the surface of the crystal (HRTEM images in the Supporting Information, Figure S1). The growth is not uniformly distributed on the surface, and there does not appear to be a preferred facet or orientation for growth. CNT formation is due to reaction with methane and not thermal decomposition; DSC/TGA analysis of Ce33Fe14B25C34 indicates this compound is stable under nitrogen to temperatures above 700 °C. The Raman spectra obtained for reaction products grown on the surface of Ce33Fe14B25C34 after varying reaction times are shown in Figure 3. The ratio of the intensities of the G-peak (graphene mode, 1565−1595 cm−1) and the D-peak (disorderinduced band at 1300 cm−1; intensity increases with number of structural defects which are commonly seen for MWCNT) is a good measure of the quality of a CNT sample; the higher the ratio, the higher the yield of SWCNT.15 Splitting of the G-band into G+ and G− modes is also indicative of SWCNT.16 Figure 3 shows the ratio of G-band to D-band intensity increasing with growth time, with G-band splitting appearing after an hour. This indicates that the initial growth on the surface of the crystal is highly disordered, including graphitic regions and B
dx.doi.org/10.1021/cm400422c | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Communication
(4) Oncel, C.; Yurum, Y. Fullerenes, Nanotubes, Carbon Nanostruct. 2006, 14, 17−37. (5) Tessonnier, J. P.; Su, D. S. ChemSusChem 2011, 4, 824−847. (6) See, C. S.; Harris, A. T. Ind. Eng. Chem. Res. 2007, 46, 997−1012. (7) Bhaviripudi, S.; Mile, E.; Steiner, S. A.; Zare, A. T.; Dresselhaus, M. S.; Belcher, A. M.; Kong, J. J. Am. Chem. Soc. 2007, 129, 1516− 1517. (8) (a) Takagi, D.; Homma, Y.; Hibino, H.; Suzuki, S.; Kobayashi, Y. Nano Lett. 2006, 6, 2642. (b) Zhou, W.; Han, Z.; Wang, J.; Zhang, Y.; Jin, Z.; Sun, X.; Zhang, Y.; Yan, C.; Li, Y. Nano Lett. 2006, 6, 2987. (9) Gao, X. P.; Qin, X.; Wu, F.; Liu, H.; Lan, Y.; Fan, S. S.; Yuan, H. T.; Song, D. Y.; Shen, P. W. Chem. Phys. Lett. 2000, 327, 271−276. (10) Zhao, H.; Bradford, P. D.; Wang, X.; Liu, W.; Luo, T. J.; Jia, Q.; Zhu, Y.; Yuan, F. G. Mater. Lett. 2010, 64, 1947−1950. (11) Ma, X.; Chen, B.; Latturner, S. E. Inorg. Chem. 2012, 51, 6089− 6095. (12) Benbow, E. M.; Dalal, N. S.; Latturner, S. E. J. Am. Chem. Soc. 2009, 131, 3349−3354. (13) Tucker, P. C.; Nyffeler, J.; Chen, B.; Ozarowski, A.; Stillwell, R.; Latturner, S. E. J. Am. Chem. Soc. 2012, 134, 12138−12148. (14) Benbow, E. M.; Dalal, N. S.; Latturner, S. E. J. Solid State Chem. 2009, 182, 3055−3062. (15) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep. 2005, 409, 47−99. (16) Jorio, A.; Pimenta, M. A.; Filho, A. G. S.; Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. New J. Phys. 2003, 5, 139. (17) Datsyuk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Carbon 2008, 46, 833−840. (18) Inorganic Crystal Structure Database, version 1.8.1; Fachinformationszentrum Karlsruhe: Karlsruhe, Germany, 2011. (19) Kalichak, Y. M.; Bodak, O. I.; Gladyshevskii, E. I. Inorg. Mater. 1976, 12, 961−965. (20) Gozzi, D.; Latini, A.; Tomellini, M. J. Phys. Chem. C 2009, 113, 45−53. (21) Book, D.; Harris, I. R. J. Alloys Compd. 1995, 221, 187−192. (22) Xing, M.; Han, J.; Lin, Z.; Wan, F.; Liu, S.; Wang, C.; Xu, Q.; Yang, J.; Yang, Y. J. Magn. Magn. Mater. 2013, 326, 201−204. (23) Davaasuren, B.; Borrmann, H.; Dashjav, E.; Kreiner, G.; Widom, M.; Schnelle, W.; Wagner, F. R.; Kniep, R. Angew. Chem., Int. Ed. 2010, 49, 5688−5692. (24) Goerens, C.; Brgoch, J.; Miller, G. J.; Fokwa, B. P. T. Inorg. Chem. 2011, 50, 6289−6296.
amorphization and disproportionation to form CeFe3 and CeHx. Disproportionation upon reaction with hydrogen is wellknown in polar intermetallics such as Nd2Fe14B and Sm2Fe17. The hydrogenation−disproportionation−desorption−recombination (HDDR) process is used to improve magnetic properties of Nd2Fe14B. Exposure to hydrogen at 650 °C induces formation of α-Fe, Fe2B, and lamellae of NdHx.21 Subsequent high temperature annealing to desorb the hydrogen reforms the Nd2Fe14B phase, but the anisotropic crystal texture is maintained, leading to improved magnetic coercivity.21,22 The simultaneous occurrence of CNT growth and hydrogenation-disproportionation in one compound is unprecedented. The only other phase studied in this work that interacts with methane to form CNT is Ce21Fe8Si7C12; the reaction at 690 °C produces MWCNT as indicated in SEM and TEM images and Raman spectra (see Figures S3 and S4 in the Supporting Information). Y5Mg5Fe4Al12Si6 appears to react to form amorphous carbon on the surface of the crystal (see Figures S4 and S5 in the Supporting Information). No growth was seen for either of these compounds at a lower temperature of 590 °C (see Figure S6 in the Supporting Information). Bulk iron and La6Fe10Al3Si were unreactive toward methane at all temperatures studied in this work. The bulk intermetallic phases that produce CNT upon reaction with methane, Ce33Fe14B25C34 and Ce21Fe8Si7C12, both have structures featuring small 0-D iron clusters capped by carbon. Bulk compounds with no Fe−Fe bonding (such as Y5Mg5Fe4Al12Si6) or with iron building blocks of two dimensions or higher (the 2D slabs in La6Fe10Al3Si, or 3D connectivity of bulk iron), do not react to form CNT. Additional studies are warranted on other phases with lowdimensional iron building blocks, although such compounds are relatively rare. Phases such as Er15Fe8C25 and Ce33Fe13B18C34 (which contain Fe6 and Fe13 clusters, respectively)23,13 and Ti7Fe4Ru18B8 (which features 1D chains and ladders of iron),24 may also react with methane to form carbon nanotubes.
■
ASSOCIATED CONTENT
S Supporting Information *
Experimental details, synthesis of Ce21Fe8Si7C12, additional SEM and TEM images, and XPS data. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work made use of the SEM, TEM, and XPS facilities of the Department of Physics and Department of Biology at Florida State University. Financial support from the NSF (Grant DMR11-06150) is gratefully acknowledged.
■
REFERENCES
(1) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’k, Y. K. Carbon 2006, 44, 1624−1652. (2) Avouris, P. Acc. Chem. Res. 2002, 35, 1026−1034. (3) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P., Eds.; Carbon Nanotubes: Synthesis, Structure, and Applications. Topics in Applied Physics 80; Springer: Berlin, 2001. C
dx.doi.org/10.1021/cm400422c | Chem. Mater. XXXX, XXX, XXX−XXX
